%--------------------------
 % @Author: Jingqiao Hu
 % @Date: 2021-01-26 15:51:34
 % @LastEditTime: 2021-01-27 11:21:56

 % boundarys: [bottom, right, up, left]
%--------------------------
function [boundarys, inner] = micro_boundary(nelx)

    nodenrs = reshape(1:(1 + nelx) * (1 + nelx), 1 + nelx, 1 + nelx);    
    alldofs = 1:2 * (nelx + 1) * (nelx + 1);

    nbottom = nodenrs(end, :);
    dbottom = reshape([2 * nbottom - 1; 2 * nbottom], 1, []);

    nright = flip(nodenrs(1:end-1, end))'; % NOTE: REVERSE!
    dright = reshape([2 * nright - 1; 2 * nright], 1, []);

    nup = flip(nodenrs(1, 1:end-1)); % NOTE: REVERSE!
    dup = reshape([2 * nup - 1; 2 * nup], 1, []);

    nleft = nodenrs(2:end - 1, 1)';
    dleft = reshape([2 * nleft - 1; 2 * nleft], 1, []);
    
    boundarys = [dbottom, dright, dup, dleft];
    inner = setdiff(alldofs, boundarys);
end